Bipolar membranes with fluid distribution passages

ABSTRACT

The present invention provides a bipolar membrane and methods for making and using the membrane. The bipolar membrane comprises a cation-selective region, an anion-selective region, an interfacial region between the anion-selective region and the cation-selective region, and means for delivering fluid directly into the interfacial region. The means for delivering fluid includes passages that may comprise a fluid-permeable material, a wicking material, an open passage disposed within the membrane or some combination thereof. The passages may be provided in many shapes, sizes and configurations, but preferably deliver fluid directly to the interfacial region so that the rate of electrodialysis is no longer limited by the diffusion of fluid through the cation- or anion-selective regions to the interfacial region.

This invention was made with government support under contractNAS9-19474 awarded by NASA. The government has certain rights in thisinvention.

FIELD OF THE INVENTION

The present invention relates to bipolar membranes and methods of makingand using bipolar membranes.

BACKGROUND OF THE RELATED ART

The production of acids and bases from their salts may be achieved usingan electrically driven process including a bipolar membrane and anaqueous medium. The process is represented by the equation: ##EQU1##

To effect and maintain separation of the various species, ion exchangemembranes are used. The most crucial of these membranes is the bipolarmembrane, so called because it is composed of regions that are selectiveto ions of opposite charges. Under the influence of an applied directcurrent, such a sandwich membrane is capable of forcibly dissociatingwater to form equivalent amounts of hydrogen and hydroxyl ions. Used inconjunction with other cation-selective and anion-selective (i.e.,monopolar) membranes, the assembly constitutes a water splittingapparatus that generates acid and base.

The basic structure of bipolar membranes is shown in FIG. 1. Bipolarmembranes include a cation selective layer, an anion selective layer anda thin interfacial region where the two ion exchange layers are incontact. The interfacial region may be either a plane or surface ofcontact or a layer itself, perhaps including adhesives or resin layersthat help bring the cation and anion exchange layers into contact. Intypical bipolar membranes, the cation selective layer, the anionselective layer and the interfacial region will be continuous and ofessentially uniform thickness. The interfacial plane or layer is veryimportant because it is where reactions, such as water or salt splittingreactions, take place under the influence of an applied electric field.However, before the reactions can take place, the species being splitmust move from outside the membrane through either the cation or anionexchange layers and into the interfacial region. Bipolar membranes ofthis or similar construction have been used as diaphragms in theelectrolysis of water to hydrogen and oxygen or as separating membranesused in reclaiming acids and alkalis from aqueous solutions of salts.

Bipolar membranes behave anisotropically under the influence of anelectric field as is illustrated by the transport process shown inFIG. 1. When a current is passed across the bipolar A membrane with itscation selective side facing toward the anode and a salt solutiondisposed on either side of the bipolar membrane, cations and anions aretransported to the interfacial region through the permselectivemembranes. However, the passage of ions out of the interfacial region islimited since the ions may not pass through regions of the bipolarmembrane having the wrong selectivity. Consequently, salt builds up atthe interfacial region and causes a low electrical resistance in theinterfacial region. When the orientation of the membrane is reversed, asshown in FIG. 2 with the cation selective side facing toward thecathode, and current is passed, salt from the interfacial region istransported to the external solution leaving only H⁺ and OH⁻ ions fromthe dissociation of water to carry the current. With this orientation ofthe membrane, the electrical resistance of the interfacial region canbecome high since water has a low conductivity. Alternatively, bipolarmembranes may be used in accordance with FIG. 2 to split water and salt.If the interfacial region is made very thin, the resistance of theinterfacial region-can be small and the membrane may be used to generateacid and base.

One of the most important parameters for the design of processes forelectrodialysis of an acid and base is the electrical resistanceexhibited by the membrane. Significant limitations are placed on theamount of current that can be applied across a bipolar membrane. Forexample, in water splitting, H⁺ and OH⁻ ions generated at the interfaceor interfacial region of the bipolar membrane require water for theirformation. This water must diffuse to the interfacial region througheither the cation selective layer or the anion selective layer. Inaddition, as the ions move to the base and acid compartments on eitherside of the bipolar membrane, the ions remove additional water from themembrane as water of hydration. As the current density is increased, therate of water removal at the interface and throughout the membrane isincreased. If the transport of water into the interfacial region,through either or both of the cation- and anion-selective layers fromthe adjacent solution, is not as rapid as the removal of water away fromthe interfacial region, by consumption and water of hydration, then somepoints of the interfacial region will dry out causing the watersplitting process to slow down. Dry spots in the bipolar membrane willcause an even higher current density over that portion of theinterfacial region that remains hydrated. Furthermore, the drying out ofthe interfacial region which results when the current density is toohigh can lead to irreversible damage to the membrane which manifestsitself in still higher electrical resistance. This in turn increases theamount of energy required to drive the process.

One exemplary method for producing bipolar membranes discloses that acation-exchange membrane and an anion-exchange membrane can be laminatedusing a mixture of polyethyleneimine and epichlorohydrin to bond themembranes to each other by curing (Japanese Patent Publication No.32-3962). A second method discloses the bonding of a cation-exchangemembrane to an anion-exchange membrane by using an adhesive having theproperties of exchanging ions (Japanese Patent Publication No. 3403961).A third method discloses a pasty material comprising vinyl pyridine andan epoxy compound that is coated on the surface of a cation-exchangemembrane, followed by exposure to radiation to obtain the bipolarmembrane (Japanese Patent Publication No. 338-16633). A fourth methoddiscloses a sulfonic acid polymeric electrolyte and an allylamine thatare adhered to the surface of an anion-exchange membrane, followed byexposure to ionizing radiation (Japanese Patent Publication No.51-4113). Yet another method discloses a process in which a polyethylenefilm is impregnated with styrene and divinylbenzene followed bypolymerization to give a sheet-like material. The sheet-like material isthen nipped between frames made of stainless steel, where one sidethereof is sulfonated, and thereafter, the sheet is detached and theremaining side is chloromethylated followed by treatment for amination(U.S. Pat. No. 3,562,139). However, these bipolar membranes exhibitinherently poor current efficiency and high-power consumption. Forexample, the use of these bipolar membranes to split water requiresapplication of a membrane potential (e.g. 2.5 V to 3.0 V, or higher)that is much higher than the theoretical water-splitting membranepotential (0.83 V).

Bipolar membranes have also been prepared by coating the mating surfacesof cation-and anion-exchange membranes with a solution comprising atleast one kind of inorganic electrolyte, such as sodium tungstate,chromium nitrate, sodium metasilicate, and ruthenium trichloride. Themating surfaces are placed in contact and pressed to give a bipolarmembrane having a low water-splitting membrane potential. This bipolarmembrane, compared with the bipolar membranes discussed above, ischaracteristic of a low water-splitting membrane potential. However, thewater-splitting potential of this membrane increases over a relativelyshort period of use due to the development of bubbles or blisters at theinterface between the cation-exchange membrane and anion-exchangemembrane. Partially or entirely separated membranes are rendereduseless. Furthermore, this membrane still provides low currentefficiency and is not satisfactory for use on an industrial scale.

Bipolar membranes which are used, for example, for isolating acids orbases from their salts by electrodialysis are ion exchange membraneshaving fixed cations on one side and corresponding anions on the otherside. Bipolar membranes may be produced, for example, by firmlyanchoring cationic or anionic groups to both sides of a neutral membraneby means of a chemical treatment (U.S. Pat. No. 4,0557,481) or bringingan anion-exchange membrane into close contact with a cation-exchangemembrane, for example by pressing the membranes on top of one another inthe presence of heat (British Pat. No. 1,038,777).

Attempts have been made to increase the stability of such bipolarmembranes, obtained by combining anion-exchange membranes withcation-exchange membranes, by applying an ion-permeable adhesive betweenthe two membranes. A polymerizable mixture of polyethyleneimine andepichlorohydrin (U.S. Pat. No. 2,829,095) or polyvinyl chloride andpolyvinyl alcohol (Israel Journal of Chemistry, 9(1971),485) has beenproposed as an adhesive. It has also been found that bipolar membranessuitable for electrodialysis may be obtained from an anion-exchangemembrane, a cation-exchange membrane and an ion-permeable adhesivecomprising of an aqueous solution of a polyvinylamine.

Bipolar membranes are difficult to produce by conventional methods. Forexample, in a chemical treatment of the surface, the two layers musthave uniform thickness and must be in contact with one another over theentire surface area in order to ensure the current flow. On the otherhand, the layers must not penetrate one another since the membrane wouldthen lose its bipolar selectivity. Although combining two monopolarmembranes gives bipolar membranes possessing defined anionic andcationic layers, this method gives rise to difficulties at the contactsurface. If the membranes are not completely in contact, the resistanceincreases. The same applies where the adhesive is not sufficientlyconductive. Moreover, very undesirable tears or bubbles may form at thepoints of contact in bipolar membranes of the stated type under typicaloperating conditions.

Bipolar membranes which consist of two individual membranes andpolyvinyl alcohol as an adhesive may be prepared by a method in whichthe cation-exchange films and the anion-exchange films are coated with apolyvinyl alcohol solution. The cation-exchange and anion-exchange filmsare laid one on top of the other and heated for about one hour at about60° C. The bipolar membrane is then dried and compressed for about 30minutes at about 100° C. Although the resulting bipolar membranesexhibit firm adhesion, their swellability in aqueous salt solutions isirreversibly restricted, and these bipolar membranes, which possessrectifying properties, are therefore unsuitable for electrodialysis.

Despite certain advances described above, the performance of bipolarmembranes is still limited by the transport of water into theinterfacial region. U.S. Pat. No. 4,851,100 proposes to increase watertransport to the interfacial region by using a continuous layer of acation-selective material that is sufficiently thin to reduce thedistance the water must diffuse to reach the interfacial region. Thisbipolar membrane is made by affixing a thin castable cation exchangemembrane to a defined base anion exchange membrane. While a bipolarmembrane of this construction might provide some increase in the rate ofwater transport to the interfacial region, the water transport rate isstill limited because it has to pass through an ion-selective layer.

Therefore, there is a need for a bipolar membrane that provides forimproved communication of fluids to the interfacial region between thecation-selective layer and the anion-selective layer. It would bedesirable if the bipolar membrane provided direct communication offluids to the interfacial region. It would be further desirable if thebipolar member were stable and exhibited low electrical resistance.

SUMMARY OF THE INVENTION

The present invention provides a bipolar membrane, comprising: ananion-selective region, a cation-selective region, an interfacial regionbetween the anion-selective region and the cation-selective region, andmeans for delivering a fluid directly into the interfacial region.

The present invention also provides a bipolar membrane comprising acation-selective region, an anion-selective region in contact with thecation-selective region, and a water distribution passage within themembrane. The contact between the cation-selective region and theanion-selective region defines an interfacial region. The waterdistribution passage may be formed in the interfacial region, theanion-selective region or the cation-selective region. Furthermore, thewater distribution passage may be open and substantially unobstructed orthe water distribution passage may comprise water-permeable material ora water wicking material.

Another aspect of the invention provides an apparatus comprising ananode, a cathode and a bipolar membrane having a cation-selectiveregion, an anion-selective region contacting the cation-selective regionand a water distribution passage within the membrane. The apparatus maybe arranged so that the anion-selective region of the bipolar membranefaces in the direction of the anode and the cation-selective region ofthe bipolar membrane faces in the direction of the cathode. In oneembodiment, a source of an electrolyte solution, such as a saltsolution, is placed in fluid communication with the water distributionpassage. The apparatus may comprise a plurality of bipolar membranes. Inanother embodiment, the apparatus may comprise a source of a saltsolution in fluid communication with a first plurality of bipolarmembranes and a source of water in fluid communication with a secondplurality of bipolar membranes, wherein the first and second pluralitiesof bipolar membranes are disposed in an alternating sequence. Otherembodiments may provide a cation-selective membrane disposed adjacentthe anion-selective region of the bipolar membrane and/or ananion-selective membrane disposed adjacent the cation-selective regionof the bipolar membrane. Alternatively, the cation-selective region ofthe bipolar membrane may face in the direction of the anode and theanion-selective region of the bipolar membrane face in the direction ofthe cathode.

Another aspect of the invention provides a method for producing amembrane with internal passages comprising the steps of hot pressing acation conducting material and an anion conducting material around aplurality of removable elements at sufficient temperature and pressureto fuse the material into a single bipolar membrane and removing theelements from the membrane to leave a passage for fluids.

Yet another aspect of the invention provides an alternative method forforming a bipolar membrane with internal passages that comprisesapplying a thin recast film of a dilute solution of an anion conductingmaterial onto a cation conducting material with tubes therein andallowing the recast film to dry. It may be desirable to apply multiplelayers of the recast film with drying between each application. Afterthe last application of recast film, the tubulated membrane is dried forseveral hours at room temperature, then baked in nitrogen at 100° C. forabout one hour to cure the recast film so that it does not return tosolution when exposed to water.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features and advantages ofthe present invention are attained can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe had by reference to the embodiments thereof which are illustrated inthe appended drawings. It is to be noted, however, that the appendeddrawings illustrate only typical embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

FIG. 1 is a cross-sectional side view of a bipolar membrane having acation-selective layer facing the anode and an anion-selective layerfacing the cathode.

FIG. 2 is a cross-sectional side view of a bipolar membrane having acation-selective layer facing the cathode and an anion-selective layerfacing the anode for salt splitting or water splitting.

FIG. 3 is a schematic perspective view of a bipolar membrane havingfluid distribution passages in accordance with the present invention,where the membrane is being used in a water desalination cell.

FIGS. 4(a) and 4(b) are schematic top and cross-sectional views of atensioning frame, respectively.

FIG. 5 is a schematic diagram of a stack of the water desalination cellsof FIG. 3.

FIG. 6(a) is a water desalination unit where the ions are collected as aconcentrated liquid.

FIG. 6(b) is a deionization unit where the ions removed from the feedsolution are collected on ion exchange resin beds within theconcentrating compartments.

FIG. 6(c) is a deionization unit where the ions removed from the feedsolution are collected on ion exchange resin beds outside the unit.

FIG. 6(d) is an arrangement showing how the ion exchange beds of FIG.6(c) may be regenerated.

FIG. 7 is a schematic diagram of an electrodialysis apparatus for watersplitting and acid/base generation.

FIG. 8 is a schematic diagram of a two compartment electrodialysisapparatus for water splitting and acid/base generation.

FIG. 9 is a schematic diagram of a double bipolar cell.

FIG. 10 is a schematic diagram of an acid/base generation systemutilizing bipolar membranes with fluid distribution passages, whereinalternating passages are in fluid communication with a salt solution andwater.

FIG. 11 is a schematic diagram of a bipolar membrane illustrating theability to produce a highly deionized solution.

FIG. 12 is a schematic diagram of an apparatus have a plurality ofalternating cation-selective and anion-selective regions with passagestherebetween for removing impurities from water.

FIG. 13 is a schematic diagram of an apparatus have a plurality ofalternating cation-selective and anion-selective regions with passagestherebetween for generating acids and bases.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a bipolar membrane and methods for makingand using the membrane. The bipolar membrane comprises acation-selective region, an anion-selective region, an interfacialregion between the anion-selective region and the cation-selectiveregion, and means for delivering fluid directly into the interfacialregion. The means for delivering fluid includes passages that maycomprise a water-permeable material, a water-wicking material, an openpassage disposed within the membrane or some combination thereof. Thepassages may be provided in many shapes, sizes and configurations, butpreferably deliver fluid directly to the interfacial region so that therate of electrodialysis is no longer limited by the diffusion of fluidthrough the cation- or anion-selective regions to the interfacialregion. In many electrochemical systems, direct feed of requiredreactants to the membrane allows for a simpler system.

While the passage(s) may extend through any portion of the membrane,including entirely within the anion-selective region or entirely withinthe cation-selective region, it is-preferred that the passage wallsprovide fluid communication with both the anion-conducting andcation-conducting regions, most preferably by having the passage extendalong the interface between the regions. In certain applications it maybe desirable for the passages to be partially or completely offsettoward either the anion-conducting and cation-conducting regions. Forexample, where the anion-selective membrane has a greater tendency todry out, the passage may be offset to have greater contact with theanion-selective region in order to increase the degree of hydration.

In another aspect of the invention, a method is provided for forming abipolar membrane, comprising the steps of providing a cation-selectivemember and an anion-selective member, disposing an adhesive and aplurality of elements between the members, and pressing the memberstogether. The element may comprises a fluid-permeable material, awicking material or a removable element, such as niobium wire and PTFEtubing, which leaves an open passage therein.

Yet another aspect of the invention provides an alternative method offorming a bipolar membrane comprising applying a thin recast film of adilute solution of an anion (cation) conducting material onto a cation(anion) conducting material with tubes therein and allowing the recastfilm to dry. It may be desirable to apply multiple layers of the recastfilm with drying between each application. After the last application ofrecast film, the tubulated membrane is preferably dried for severalhours at room temperature, then baked in the presence of nitrogen at100° C. for about one hour to cure the recast film so that it does notreturn to solution when exposed to water.

In another aspect of the invention, a method is provided for producing amembrane with internal passages. The method comprises hot pressing acation conducting material and an anion conducting material around aplurality of removable elements at sufficient temperature and pressureto fuse the material into a single bipolar membrane and removing theelements from the membrane to leave a passage for fluids.

A further aspect of the invention provides a membrane comprising aplurality of alternating cation-selective regions and anion-selectiveregions, each region disposed in contact with adjacent regions, andfluid distribution passages within at least two of the regions. In thismanner, the fluid distribution passages may define compartments,analogous to those of conventional electrodialytic processes, that arecontained within an integrated membrane. For example, the passage(s)between a first cation-selective region and a first anion-selectiveregion may form a diluting compartment while the passage(s) betweeneither a second cation-selective region and the first anion-selectiveregion or a second anion-selective region and the first cation-selectiveregion may form a concentrating compartment. It should be recognizedthat the fluid distribution passages may operate in either a fluiddelivery mode or a fluid collection or removal mode. Other arrangementsand processes will become apparent those in the art given the disclosureherein.

Additionally, the fluid distribution passages of the present bipolarmembranes may comprise open channels, elements (such as fluid absorbentstrands or wicks), networks of channels or elements, or combinationsthereof.

Membranes having internal fluid passages in accordance with the presentinvention can be formed in number of ways. One preferred method ofmaking tubulated membranes uses an apparatus comprising a pair of uppertensioning rods in a spaced relation having top surfaces lying in acommon plane and a pair of lower tensioning rods separated by the uppertensioning rods having top surfaces lying in or below the common planeof the upper tensioning rods. The upper and lower tensioning rods arecoupled to a securing member, such as by bolting both ends of each rodto a pair of opposing, rigid members. One or more removable elements aredisposed over the pair of upper tensioning rods and around one of thelower tensioning rods to define the size and spacing of the passages. Itis preferred that only one removable element be used, such as a longPTFE fiber, and that the element be wound around the rods a multiplenumber of times.

The bipolar membranes of the present invention may be produced usingconventional ion exchange membranes. For example, bipolar membranes maybe produced by copolymerization of styrene and divinylbenzene orbutadiene, or the copolymerization of acrylonitrile and butadiene,wherein the cations are firmly attached to the membrane by, for example,sulfochlorination, and the anions are firmly attached to the membrane bychloromethylation and reaction with tertiary amines. The thickness ofthe bipolar membranes is preferably between about 0.1 and about 1 mmthick. Furthermore, the bipolar membranes may optionally include areinforcing material of various kinds and forms depending on the methodin which cation-exchange membranes are prepared.

The bipolar membrane of the present invention may be made with anyconventional cation-exchange membrane, including membranes having anion-exchange group such as a sulfonic acid group or a carboxylic acidgroup. The cation-selective region can be made from a material selectedfrom partially or fully fluoronated polymers having carboxylic orsulfonic acid functional groups, copolymers of ethylene with acrylicacid, copolymers of ethylene with methacrylic acid, styrene polymershaving carboxylic acid functional groups or sulfonic acid functionalgroups, divinylbenzene polymers having carbexylic acid functional groupsor sulfonic acid functional groups, derivatives thereof and mixturesthereof. The most preferred cation-exchange membranes include a sulfonicacid group that retains an exchange group even under an acidiccondition. Furthermore, because the cation-exchange membrane is notrequired to have any particular capacity, most conventional membranesare suitable. The cation-exchange membrane will preferably have acapacity between about 0.5 and about 3 milli-equivalents per gram(meq/g), most preferably between about 1 and about 2.5 meq/g. Thecation-exchange membrane may be a polymerized type, a homogeneous typeor a nonhomogeneous type. Furthermore, the cation-exchange membrane mayinclude a small amount of an anion-exchange group so long as they havecation transport numbers of not less than about 0.9.

The anion exchange layer may be made with any conventional anionexchange material having ion-exchange groups such as positively chargedorganic ions, amino or quaternary ammonium groups. The anion-selectiveregion can be made from a material selected from polymers havingpartially or fully fluoronated primary chains (backbones), saturatedhydrocarbon primary chains, partially unsaturated primary chains,aromatic or partially aromatic primary chains, or saturated primarychains containing hetero atoms with functional groups selected frompositively charged functionalities, amino groups, amine groups, andderivatives thereof. The polymeric membrane structure would contain theanion-exchange group locked in the organic network. The polymer can be acopolymer of vinylpyridine, divinylbenzene with monomers styrene,ethylene, methacrylic acid or propylene copolymerized in variousamounts. The anion exchange membrane can have a reinforcing matrix,which may include polyethylene, polypropylene, polyvinylchloride andpolyvinyl acetate. The anion-exchange membrane will preferably have acapacity between about 1 and about 3 milli-equivalents per gram (meq/g).The anion exchange membrane may be a polymerizable type, a homogeneoustype or a non-homogeneous type.

In accordance with the invention, the ion exchange membranes arepreferably secured together using an adhesive, such as an "ionicadhesive" made up of positively and negatively charged ion conductingspecies. These adhesives include, but are not limited to,epichlorohydrin, polyethylenimine, polyacrylic acid, polyvinylamine,poly (4-vinyl) pyridine, powdered commercial anion and cation exchangeresin, and combinations thereof. After applying the ionic glue, thecation conducting material and the anion conducting material arepreferably hot pressed around a plurality of removable elements atsufficient temperature and pressure to bond the material into a bipolarmembrane. The removable elements may be removed by extraction ordissolution leaving a passage for fluids. A preferred adhesive is anaqueous solution containing a mixture of polyacrylic acid andpolyethylenimine, most preferably in a polyethylenimine:polyacrylic acidratio of about 6:1.

Alternatively, the adhesive may include a polyvinylamine in which theamino group is substituted with an alkyl group having from 1 to 4 carbonatoms and the polyvinylamine has a molecular weight between about 10⁴and about 10⁶. The concentration of the aqueous polyvinylamine solutionmay be between about 0.5 and about 70 percent by weight, but thepreferred concentration is between about 3 and about 15 percent byweight. Solutions of the aqueous polyvinylamine may be obtained, forexample, by a conventional method of acidic or alkaline hydrolysis ofpolyvinylformamide or of polyvinylacetamide with sodium hydroxidesolution or hydrochloric acid. A preferred method of forming an aqueouspolyvinylamine solution includes hydrolyzing aqueous polyvinylformamidewith hydrochloric acid at a temperature between about 60° C. and about100° C. The aqueous polyvinyl formamide concentration is preferablybetween about 1 and about 50 percent by weight, most preferably betweenabout5 and about 20 percent by weight. The resulting polyvinylaminesolutions are still liquid and can be easily applied onto the membranes.

The adhesive solutions may be applied using any conventional technique,including brushing or roller-coating onto one or both of the ionexchange membranes. The solution is preferably applied at a temperaturebetween about 10° C. and about 50° C. It is also possible to impregnatethe membranes on both sides with the solution. However, the outermembrane surface is preferably washed free of the adhesive duringfinishing of the bipolar membrane. The thickness of the adhesive layeris preferably between about 0.001 and about 0.05 mm.

In the bipolar membrane of the present invention, the cation-exchangemembrane may be bonded to the anion-exchange membrane by any method.However, it is preferred that the cation-exchange membrane and theanion-exchange membrane be adhered closely to each other at a peelstrength of not less than 0.2 kg·f/25 mm in a wet state to preventseparation of both the membranes when the bipolar membrane is used in awet state, such as in water splitting. A bipolar membrane with a lowpeel strength will allow bubbles or blisters to form at the interfacebetween the anion-conducting membrane and the cation-conducting membraneduring use. Bubbles and blisters cause a reduction in current efficiencyper unit membrane surface area and a gradual increase, over long periodsof use, in membrane potential. Such membranes must be periodicallyreplaced.

FIG. 3 is a schematic perspective view of a tubulated bipolar membranebeing used in a water desalination cell. The bipolar membrane separatingthe anode and cathode serves the dual purposes of conducting cations tothe cathode and anions to the anode. The bipolar membrane of the presentinvention provides passages having a cross-sectional dimension betweenabout 2 and about 9 mils (between about 0.002 inches about 51 μm! andabout 0.009 inches about 229 μm!) within the bipolar membrane. In thismanner, water can be provided to the open ends of the passages along oneedge of the membrane and delivered throughout the membrane by capillaryaction. The water may even be circulated through the passages and exitthe membrane at the open ends.

The tubulated bipolar membrane can be formed in a variety of ways.Briefly, the technique involves pressing anion and cation conductingmaterials around a plurality of removable elements at sufficienttemperature and pressure to fuse the material into a single membrane.After the material is fused the elements are removed from the membraneto leave a passage for fluid. The removable elements may take any shapeor form so long as the passage provide a substantially uniform flow offluid throughout the entire membrane. The preferred removable elementsare substantially parallel wires or tubes. However, it may be possibleto form the passages around elements which are later removed throughdissolution.

FIGS. 4(a) and 4(b) are schematic top and cross-sectional views of atensioning frame 130 that is designed for holding a series removableelements tight and parallel during formation of a membrane with internalpassages. This tensioning frame 130 has a pair of upper tensioning rods132 having top surfaces lying in a common plane (shown best in FIG. 4(b)at line 134). The upper tensioning rods 132 are a sufficient distanceapart to form a membrane therebetween, typically from about 3 to about 5inches. A pair of lower tensioning rods 136 are separated by the uppertensioning rods 132 having top surfaces lying in or below the commonplane of the upper tensioning rods 132. These two sets of tensioningrods are firmly held in position by two substantially parallel sidebars138.

One end of a long removable element 140, such as string, wire or tubing,is tied to a lower tensioning rod 136, say at point 142. The string isthen passed over the top of both upper tensioning rods 132 and aroundthe other lower tensioning rod 136. The string is threaded back andforth in this fashion, positioning the wire in a spaced arrangement,until the wire covers a planar area the size of the desired membrane,typically between a 3 inch by 3 inch square and a 5 inch by 5 inchsquare. However, the completed bipolar membrane may be formed or cutinto any shape. Once the membrane has been formed, the membrane can beremoved from the frame either by cutting the tubing and/or wires alongboth ends near the upper or lower tensioning rods, 132 or 136respectively, or by disconnecting the rods 132 and 136 respectively, orby disconnecting the rods 132 and 136 from one sidebar 138 by removingbolts 144.

The apparatus may also include means for adjusting the tension on thewire that is threaded over the tensioning frame. The tension is adjustedby moving (1) an upper tensioning rod upward or outward, and/or (2) alower tensioning rod downward or outward. Typically, the rods are movedand the elements tightened by turning a set screw 146 in the sidebar 138which puts an angular force on the rod. It is preferred that thetensioning frame have only one adjustable rod and that the sidebars havea narrow slot 148 through which the rod can travel as the screw 146 istightened.

A bipolar membrane is formed by placing an anion-exchange membrane and acation-exchange membrane on either side of the elements and hotpressing. The membrane is hot pressed by placing a block, preferably ametal such as aluminum, small enough to fit in the space inside theframe on the lower platen of the press (not shown) and topping it with aload leveling pad, preferably silicone rubber, and a release sheet,preferably a PTFE finished cloth. A first membrane is placed on therelease sheet. Next, the filled tensioning frame is placed with theremovable elements directly over the first membrane. A second membraneis then placed on top of the filled frame. This arrangement is toppedwith a second release sheet, a second leveling pad, and a second block.The press is then closed and the press cycle carried out.

If it is desirable to form a membrane and electrode assembly, it ispossible to combine the formation of the internal passages and the hotpressing of membrane and electrode assembly into a single press step.The press package is assembled in a manner analogous to that describedfor the pressing of membranes, except that the electrodes are positionedon either side of the membrane materials. The press conditions aredetermined by the configuration of membrane material, as set out above.For example, when hot pressing a membrane and electrode assembly havinganion and cation membranes above and below PTFE tubes, the preferredpress conditions are a pressure of about 400 psi at a temperature ofabout 200° C. for about one minute. One further optional component whichcan be added to the press package for hot pressing is a PTFE gasketpositioned around the perimeter of the assembly. This complex assemblyis then ready for installation in an electrolytic cell.

The bipolar membranes of the present invention are useful in manyelectrodialysis methods and apparatus, wherein fluid is introduceddirectly into the interfacial region or zone of the membrane as shown inFIGS. 5 through 11. Electrodialysis apparatus in accordance with thepresent invention may comprise a single cell unit or a plurality of cellunits, frequently referred to as a "stack", and each cell may comprisemultiple compartments. In the most general case, a three compartmentunit cell can be used.

FIG. 7 is a schematic diagram of an electrodialysis apparatus for watersplitting and acid/base generation having a cation-selective membrane, abipolar membrane and an anion-selective membrane. These membranes definecompartments there between and maintain separation of fluids withinthose compartments, such as the acid, base and salt solutions shown. Thethree compartments and three membranes constitute a cell unit. A largenumber of cell units, frequently between about 30 and about 200 of suchcell units or cells, may be assembled to form an electrodialysis stack.Typically, such a stack will have only a single set of electrodesdisposed at each end of the stack or single cell unit.

If a salt solution is introduced into salt compartments on the anodeside of a cation-selective membrane and on the cathode side of ananion-selective membrane and an electrical potential is establishedbetween the anode and cathode, then cations in the salt solution willmigrate towards the anode and anions in the salt solution will migratetowards the cathode. The cations will permeate the cation-exchangemembrane into the base compartment and form a base with the hydroxideions formed within the first bipolar membrane and transported towardsthe anode. On the other side of the bipolar membrane, which is directedtowards the cathode, the protons, which are formed simultaneously withthe hydroxide ions, are collected. The protons form an acid in the acidcompartment with the anions migrating from the salt solution through theanion-exchange membrane towards the anode.

The principle of electroneutrality does not permit H+ and OH- productions to exist outside the bipolar membrane without counter ions tobalance the charges. Electroneutrality is satisfied by assemblingbipolar membranes in positions between alternating monopolar anion andcation membranes. Electroneutrality is maintained within theanion-selective and cation-selective layers by immobilized ionic chargeswithin the layers.

A manifold or conduit may be disposed external to the cell or within thediaphragms or support structure of the cell in order to deliver fluid tothe fluid passages within the bipolar membranes and the compartmentsmaking up the cells. Fluid flow to any member of the cell may becontinuous or discontinuous. Furthermore, fluid may be made to enter andleave the bipolar membranes by separate conduits or the same conduits.

Fluid may also be introduced in a "dead end" arrangement, as well as a"pass through" arrangement. In many instances, water will be introducedinto the bipolar membrane in order to bring bout a water-splittingreaction. In accordance with the invention, it is preferred that waterbe supplied through a passage directly to the interfacial region of thebipolar membrane independent of water diffusing to the interfacialregion from the outside surfaces of the membrane. The water delivered tothe passage does not have to be pure, but may contain dissolvedsubstances, such as inorganic or organic salts.

The bipolar membranes of the present invention may be used to advantagein any known cell arrangements or configurations that use bipolarmembranes. FIG. 8 is a schematic diagram of a two compartmentelectrodialysis apparatus for water splitting and acid/base generation.Salts of acids like formic acid, acetic acid or lactic acid can beconverted into their acids and corresponding amounts of base in a stackwith only bipolar and cation exchange membranes. Conversely, FIG. 9shows a three compartment cell in which two bipolar membranes are used.It should be understood that there are many types of electrochemicalcells that can use bipolar membranes with internal passages, and thearrangements shown in FIGS. 5 through 11 are merely examples.

The present invention also includes an apparatus in which only thebipolar membranes of the present invention are used for acid-basegeneration. FIG. 10 is a schematic diagram of an acid/base generationsystem utilizing only bipolar membranes with fluid distribution passagesalong the interfacial region, wherein alternating passages are in fluidcommunication with a salt solution and water.

FIG. 5 is a schematic diagram of a stack of the water desalination cellsof FIG. 3. The salt is removed from the feed solution passing throughthe bipolar membranes and concentrated in the liquid passing on eitherside thereof. The chloride anions from the anion exchange side of afirst bipolar membrane are recombined with the sodium cations from thecation exchange side of a second bipolar membrane. It is preferred thatthe concentrate solution be provided in a serpentine path between aplurality of the bipolar membranes to produce a single salt concentratesolution.

The bipolar membranes of the present invention provide a uniqueapparatus for the purification of liquids by reducing the concentrationof ions or molecules in the liquid. Known techniques for purifying andisolating liquids or to obtain concentrated pools of specific ions ormolecules from a liquid mixture include liquid chromatography, membranefiltration, reverse osmosis, ion exchange and electrodeionization andelectrodialysis.

In conventional electrodialysis processes in which the cell units arecomposed of anion and cation membranes that are completely separated bya series of fluid concentrating and diluting compartments filled withsolution. A large number of such cell units can be assembled to form anelectrodialysis stack. Direct current input to the stack is made viaelectrodes at the ends of the cell. During the process, the dilutingcompartments contain feed water that is being purified gradually bydesalting (decreases in ionic content) while the concentratingcompartments become enriched. This gradual decrease of conductivity inthe diluting compartments is the decisive factor for end-product purity.Theoretically, current could pass through water until all electrolytes(ionic components) have been removed. However, the amount of powerrequired to continue this process beyond a given electrolyte level wouldbe phenomenal. In practice, purification by electrodialysis isterminated when the Total Dissolved Solids (TDS) of the treated water isequivalent to about 500 ppm as sodium chloride. Thus, conventionalelectrodialysis processes are unable to produce highly deionizedsolutions from already dilute electrolyte solutions.

However, when the bipolar membranes of the present invention aredisposed between electrodes with the anion-selective region facing thedirection of the anode and the cation-selective region facing thedirection of the cathode, a highly deionized solution may be producedwithout the use of ion exchange resins within the cell. FIG. 11 is aschematic diagram of a bipolar membrane illustrating the ability toproduce a highly deionized solution. A contaminated feed stream isintroduced into the passages of the bipolar membrane and an electricpotential is applied across the cell. Because of the semipermeabilityproperties of the membranes and the directionality of the electricalpotential gradient, negatively charged ions migrate towards the anode(positive electrode) and positively charged ions migrate towards thecathode (negative electrode). This causes the ions in the passages(i.e., diluting channels) to become depleted. These ions will becomeconcentrated in the adjacent concentrating compartments. The anodic andcathodic reactions are given in equations 1 and 2.

    Anode:2H.sub.2 O→O.sub.2 +4H.sup.+ +4e.sup.-        Equation(1)

    Cathode:2H.sub.2 O+2e.sup.- →2OH.sup.- +H.sub.2     Equation(2)

At the anode, hydrogen ions are liberated, decreasing the pH andcreating acidic conditions. The anions pass through the anion-selectiveregion, migrate to the anode and combine with the H⁺ ions to form anacid solution. At the cathode, hydroxyl ions are produced under anapplied potential. The cations pass through the cation-exchange membraneand migrate to the cathode and combine with the OH⁻ ions to form a basicsolution. Thus, the movement of counter ions across the polymermaintains charge neutrality in all compartments. The passage of about96,494 Coulombs (one Faraday) causes the transfer of one chemicalequivalent (mole) of salt. When the two solution from the concentratingcompartments are combined, brine is formed.

The selective permeability of ion-selective or ion-exchange membranes isimportant to this process. To maintain electrical neutrality, each ofthese fixed charges must be associated with an opposite charge. In thecase of a cation-selective membrane, negative charges are fixed to themembrane and are associated with a positive counter ion, such as Na⁺.Therefore, cations (the counter ion) move very easily from fixednegative group to fixed negative group, provided there is another cationwaiting to replace them. The fixed, electronegatively charged particleson the cation membrane matrix repel anions, so they cannot enter to anysignificant extent. In the case of a cation exchange membrane, negativecharges are fixed to the membrane and are associated with a positivecounter ion such as Na⁺. Ion-exchange membranes are electricallyconductive because of their ability to exchange counter ions.

Ion exchange membranes are usually impermeable to water and thereforecan function as a barrier to bulk flow while allowing transfer ofcounter ions under the influence of an electrical potential. Inpractice, ion-exchange membranes are not completely efficient and asmall percentage of co-ion permeability does occur, as does a smallpercentage of water transfer (by osmosis and/or transfer of hydratedions).

As discussed above, conventional electrodialysis processes are notefficient in producing highly deionized fluids because electrochemicaltechniques operate only on process streams that have a relatively highdissolved ion content to carry out charge transfer between theelectrodes. In other words, they only work on solution with a highelectrolyte content. However, the bipolar membranes of the presentinvention provide a continuous ionically conducting pathway for iontransfer and also serves as a conductivity bridge between thecation-selective and anion-selective regions for the movement of ions.FIG. 11 illustrates how the passage allows the feed solution to comeinto direct contact with both ion exchange regions allowing a highsurface area for ion transfer. In FIG. 11, the bipolar membrane is beingused for desalting a water stream. The bipolar membranes are useful insplitting all inorganic and organic salts and mixtures thereof.

As the reaction occurs, the fluid in the diluting channels (the bipolarmembrane passages) to become deionized. However, dissociation of waterinto OH⁻ and H⁺ (water splitting) occurs at the membrane interface whena current is applied. Therefore, even though the solution beingprocessed through the passages of the bipolar membrane may be a poorionic conductor (i.e., a dilute electrolyte), the water splittingreaction combined with the efficient transfer of H⁺ and OH⁻ ions by thefixed ion exchange sites, ensures high transmembrane ionic conductivity.The electrochemical cell's ion conducting circuit is completed by theconcentrating compartments, the space between the membrane and theelectrodes. If necessary, the conductivity of these concentratingcompartments can be enhanced by being filled with ion exchange resins.The arrangement shown in FIG. 11 permits high currents to be appliedacross the cell at low voltages. The bipolar membrane with fluiddistribution passages aids water dissociation as fluids can readilydiffuse from the tubes across the entire area of the membraneinterfacial region. The water dissociation process can occur withoutreliance on slow transport of water to the membrane's interface from theexternal surfaces of the membrane.

The bonded ion-selective membranes create a permanent and continuousionic medium between the electrodes that permits the process to operateefficiently over a wide range of electrolyte concentrations or even whenthere is no electrolyte. This process allow feed water to be purified tolevels of less than1 ppm sodium chloride with no increase in electricityconsumption.

FIG. 6(a) is a water desalination unit where the ions are collected as aconcentrated liquid. The salt is removed from the feed solution byintroducing the feed stream through the passages of the bipolarmembrane. When a voltage is applied, the anion exchange membrane allowstransfer of anions and the cation exchange membrane allows transfer ofcations. A liquid stream circulates around the anion and cation exchangemembrane and collects the ions. The cell manifolding and fluid conduitsare arranged in such a manner that anions and the cations from theconcentration compartments are combined to produce a single salt (brine)concentrate solution.

In some processes it may be desirable to combine the apparatus with ionexchange resins or particles. FIG. 6(b) is a deionization unit where theions removed from the feed solution are collected on ion exchange resinbeds packed within the concentrating compartments. The space between theanode and the bipolar membrane is packed with anion-exchange resin beadsand the space between the cation-exchange membrane and the cathode ispacked with cation exchange resin beads. The ion-exchange resin beadsserve a dual role as an absorption bed for separated ions and as animmobile electrolyte, i.e., it provides the necessary conductivity inthe region between the bipolar membrane and the electrodes for operationof the process at low cell voltages. It should be noted that in thisconfiguration, the diluting compartment is free of any ion exchangeresin material.

FIG. 6(c) is a deionization unit where the ions removed from the feedsolution are collected on ion exchange resin beds outside the unit.Fluids from the concentrating compartments are fed to external ionexchange beds connected to the anodic and cathodic fluid concentrationcompartments. Anions from the anion exchange membrane collect on theanion exchange bed. This stream can contact the resin bed in a singlepass or by recirculation. Similarly, cations from the cation side of thebipolar membrane collect on the cation exchange resin bed. Locating theion exchange beds external to the cell has a number of advantages overplacing it within the electrochemical cell. It reduces the cell size anddecreases the electrical resistance that results when there is a largespace occupied by the resin between the electrodes and membrane. Theremay be a configuration where the fluid from the concentrationcompartments contain both dissolved anions and cations. In thisembodiment, a mixed resin bed (combined anion and cation exchangeresins) may be used.

The arrangement in FIG. 6(d) shows how the ion exchange beds of FIG.6(c) may be regenerated. Under an applied voltage, hydrogen ions areproduced at the anode resulting in acidic conditions and hydroxyl ionsare produced at the cathode resulting in basic conditions. Thecompartment immediately adjacent to the anode is connected to the cationexchange resin bed so that the fluid causes the regeneration of the bedby stripping the cations from the cation exchange bed. The basicconcentrate stream from the compartment immediately adjacent the cathodeis connected to the anion exchange resin bed to regenerate the resin bystripping the anions from the bed. Both streams are collected as asingle salt concentrate solution. The fluid flow can be reconfiguredusing valves and switches to return to the configuration shown in FIG.6(c).

FIG. 12 is a schematic diagram of an apparatus have a plurality ofalternating cation-selective and anion-selective regions with passagestherebetween for removing impurities from water. FIG. 13 is a schematicdiagram of an apparatus have a plurality of alternating cation-selectiveand anion-selective regions with passages therebetween for generatingacids and bases. The apparatus of FIGS. 12 and 13 operate analogouslywith that of FIGS. 11 and 10, respectively, except that the compartmentsare now defined by passages between additional ion-selective regions.

While the foregoing is directed to the preferred embodiment of thepresent invention, other and further embodiments of the invention may bedevised without departing from the basic scope thereof, and the scopethereof is determined by the claims which follow.

What is claimed is:
 1. A bipolar membrane comprising:(a) acation-selective region; (b) an anion-selective region in contact withthe cation-selective region; and (c) a fluid distribution passage withinthe membrane.
 2. The bipolar membrane of claim 1, wherein the contactbetween the cation-selective region and the anion-selective regiondefines an interfacial region, and wherein the fluid distributionpassage is formed in the interfacial region.
 3. The bipolar membrane ofclaim 1, wherein the passage is formed in the anion-selective region. 4.The bipolar membrane of claim 1, wherein the passage is formed in thecation-selective region.
 5. The bipolar membrane of claim 1, wherein thecation-selective region comprises a material selected from partially orfully fluoronated polymers having carboxylic or sulfonic acid functionalgroups, copolymers of ethylene with acrylic acid, copolymers of ethylenewith methacrylic acid, styrene polymers having carboxylic acidfunctional groups or sulfonic acid functional groups, divinylbenzenepolymers having carboxylic acid functional groups or sulfonic acidfunctional groups, derivatives thereof or mixtures thereof.
 6. Thebipolar membrane of claim 1, wherein the anion-selective regioncomprises a material selected from polymers having partially or fullyfluoronated primary chains, saturated hydrocarbon primary chains,partially unsaturated primary chains, aromatic or partially aromaticprimary chains, or saturated primary chains containing hetero atoms withfunctional groups selected from positively charged functionalities,amino groups, amine groups, or derivatives thereof.
 7. The bipolarmembrane of claim 1, wherein the fluid distribution passage is open andsubstantially unobstructed.
 8. The bipolar membrane of claim 7, whereinthe fluid distribution passage has a cross-sectional profile that iscurvilinear.
 9. The bipolar membrane of claim 8, wherein the curvilinearcross-sectional profile is selected from ovals, circles or ellipses. 10.The bipolar membrane of claim 1, wherein the fluid distribution passagecomprises a water-permeable material.
 11. The bipolar membrane of claim1, wherein the fluid distribution passage comprises a wicking material.12. A bipolar membrane comprising a plurality of alternatingcation-selective regions and anion-selective regions, each regiondisposed in contact with adjacent regions, and fluid distributionpassages within at least two of the regions.
 13. The bipolar membrane ofclaim 12, wherein the fluid distribution passages define at least onediluting compartment and at least one concentrating compartment.
 14. Anapparatus, comprising:an anode; a cathode; and a bipolar membrane havinga cation-selective region, an anion-selective region contacting thecation-selective region and a fluid distribution passage within themembrane.
 15. The apparatus of claim 14, wherein the contact between thecation-selective region and anion-selective region defines aninterfacial region, and wherein the fluid distribution passage is formedin the interfacial region.
 16. The apparatus of claim 14, wherein thefluid distribution passage is open and substantially unobstructed. 17.The apparatus of claim 14, wherein the fluid distribution passagecomprises a fluid-permeable material.
 18. The apparatus of claim 14,wherein the fluid distribution passage comprises a wicking material. 19.The apparatus of claim 14, wherein the anion-selective region of thebipolar membrane faces in the direction of the anode and thecation-selective region of the bipolar membrane faces in the directionof the cathode.
 20. The apparatus of claim 19, further comprising aliquid source in fluid communication with the fluid distributionpassage.
 21. The apparatus of claim 20, wherein the liquid sourcecontains a salt.
 22. The apparatus of claim 20, wherein the liquidsource contains an ionic impurity.
 23. The apparatus of claim 19,comprising a plurality of bipolar membranes.
 24. The apparatus of claim19, further comprising:a liquid source containing a salt solution influid communication with a first plurality of bipolar membranes; aliquid source containing water in fluid communication with a secondplurality of bipolar membranes, wherein the first and second pluralitiesof bipolar membranes are disposed in an alternating sequence.
 25. Theapparatus of claim 19, further comprising a cation-selective membranedisposed adjacent the anion-selective region of the bipolar membrane.26. The apparatus of claim 19, further comprising an anion-selectivemembrane disposed adjacent the cation-selective region of the bipolarmembrane.
 27. The apparatus of claim 14, wherein the cation-selectiveregion of the bipolar membrane faces in the direction of the anode andthe anion-selective region of the bipolar membrane faces in thedirection of the cathode.
 28. The apparatus of claim 14, furthercomprising a source of liquid in fluid communication with the fluiddistribution passage, wherein the source of liquid is selected fromaqueous liquids, non-aqueous liquids or mixtures thereof.
 29. Theapparatus of claim 14, further comprising a compartment adjacent theanion-selective side of the bipolar membrane, wherein the compartment isin fluid communication with a resin bed.
 30. The apparatus of claim 14,further comprising a compartment adjacent the cation-selective side ofthe bipolar membrane, wherein the compartment is in fluid communicationwith a resin bed.
 31. The apparatus of claim 30, wherein the resin bedis disposed within the compartment.
 32. The apparatus of claim 30,wherein the resin bed is disposed outside the compartment.
 33. A bipolarmembrane, comprising: an anion-selective region, a cation-selectiveregion, an interfacial region between the anion-selective region and thecation-selective region, and means for delivering a fluid directly intothe interfacial region.